1. Field of the Invention
The present invention relates to a disk apparatus wherein positioning of a recording/reproduction head, such as a magnetic head, an optical pickup, or the like, is performed using an actuator, and to a control method of such a disk apparatus.
2. Description of the Prior Art
In recent years, disk apparatuses, such as magnetic disk apparatuses, and the like, have been decreasing in size and increasing in capacity. For example, the capacity of magnetic disk apparatuses has been increased by increasing the track density of magnetic disks, and the track pitch will be further reduced in the future. Therefore, in order to record/reproduce data on/from a magnetic disk, it is necessary to precisely position a magnetic head at a target track among the tracks formed at a small pitch.
In a commonly-employed magnetic disk apparatus, servo information used for positioning a magnetic head is stored in a magnetic disk in advance, and the magnetic head is positioned according to the servo information. Specifically, the position of the magnetic head is detected by reading the servo information with the magnetic head, and an error between the position of a target track and the current position of the magnetic head is determined. Then, a position error signal which indicates the position error of the magnetic head with respect to the target track is generated, and the magnetic head is positioned such that the position error signal becomes minimum.
Further, in order to increase the accuracy in positioning the magnetic head, the control frequency of a magnetic head positioning control system is set to be high, and the magnetic head is quickly positioned at a target track, such that a necessary positioning accuracy is ensured.
However, an actuator of the disk apparatus itself may have a natural vibration mode of higher-order. Thus, if the control frequency is increased for the purpose of increasing the positioning accuracy, the positioning system may become unstable due to the natural vibration. In an actual case, the band of the control frequency is limited due to a natural mechanical vibration of an actuator itself, and therefore, increasing the control frequency of the positioning system is limited.
In recent years, the effect of external force that acts against an actuator on the positioning system has been increasing along with the increase in the track density and the decrease in size and weight of the actuator. Further, demand for accurate positioning of a magnetic head has become more and more severe along with the reduction in size and the increase in recording density of the magnetic disk apparatus. In view of such, in the magnetic disk apparatus industry, a control method in which the effect of external force is considered has been receiving attention, and compensation of external force by feed-forward control has been practiced.
For example, a control method has been proposed wherein a head position signal is obtained from servo information recorded in a magnetic disk, and external force is compensated by disturbance estimation means to which the head position signal and a driving signal of an actuator are input (see, for example, Japanese Unexamined Patent Publication No. 9-231701).
Examples of external force that acts against the actuator include disturbance, such as inertial force that acts against the actuator due to the effect of external impact or vibration, bearing friction in the actuator, elastic force of a flexible print circuit (FPC) which connects the actuator and an electronic circuit board, etc. In the above conventional disk apparatus, disturbance estimation means for estimating such disturbance receives a head position signal and a driving signal of the actuator and operates based on these signals. However, servo information recorded in a magnetic disk discretely exists over the disk with a certain sampling frequency, and therefore, the head position signal is not a serial signal. Thus, the control band in which the disturbance estimation means can estimate disturbance is influenced by the sampling frequency of sector servo of a magnetic disk apparatus, and the upper limit is determined by the sampling frequency of the sector servo. This upper limit makes it difficult to precisely estimate external force that acts against the actuator. Further, the upper limit also makes it difficult to reduce the effect of the disturbance that externally acts against the actuator to a level such that the reduced effect causes no trouble in actual practice. As a result, it is difficult to control a head to always precisely follow a target track.
On the other hand, in some cases, the resistance value of a driving coil of the actuator is different among driving coils. In other cases, a driving coil emits heat when a driving current flows through the driving coil, and the resistance value thereof varies due to the increase in temperature of the coil. In such cases, the positioning system including the disturbance estimation means becomes unstable due to an error in the resistance value of the driving coil (deviation from the nominal value).
It should be noted that the above problems are not limited to magnetic disk apparatuses but are common among general disk apparatuses.